Digital coherent optical receiver presents advantages of high sensitivity, excellent electric-equalization capability, high spectrum efficiency, etc., and thus is believed to be a key technology for high speed optical communication system.
In a coherent optical receiver, a signal light is mixed with a local oscillation (LO) light generated by a local oscillator, whereby the magnitude and phase information of the signal light is transferred to a base band electric signal. An original transmit signal can be finally recovered by sampling, quantizing and digital-signal-processing the base band electric signal. It is possible to compensate almost completely the linear damage (e.g. chromatic dispersion and polarization mode dispersion, etc.) of an optical signal by electric-equalization technology, since coherent detection keeps all the information of an optical field. The damage amount (e.g. chromatic dispersion amount) that can be compensated by a digital coherent optical receiver depends solely on the scale of digital circuit (e.g. the tap number of a finite impulse response (FIR) filter). S. J. Savory et al. verified, in off-line tests, the feasibility of a coherent optical communication system without in-line dispersion compensation. They applied dispersion compensation to a polarization multiplexed 42.8 Gb/s NRZ-QPSK signal, which was transmitted 6400 km in a standard single mode fiber link, by using a FIR filter with 512 taps. The optical signal to noise ratio (OSNR) penalty of the thus compensated signal is merely 1.2 dB. (S. J. Savory, G. Gavioli, R. I. Killey, P. Bayvel, “Transmission of 42.8 Gbit/s polarization multiplexed NRZ-QPSK over 6400 km of standard fiber with no optical dispersion compensation”, OFC2007, OTuA1.)
On the other hand, as shown in FIG. 1(a), it is common that the electric equalization function is separated into an equalizer with fixed coefficient and a short FIR filter that is controlled by an adaptive algorithm. FIG. 1(a) schematically illustrates the structure of a conventional polarization diversity coherent optical receiver. A double-polarization optical signal 101 and a continuous light 103, which is generated by a local oscillation laser 102, are mixed and optical-electric converted in a polarization diversity coherent detector 104, resulting in complex base band electric signals 105 and 106 which carry the information about two polarization states of the double-polarization optical signal 101. The complex base band electric signals 105 and 106 are converted into complex digital signal sequences 109 and 110 respectively by analog-to-digital converters 107 and 108, and then are processed by a chromatic dispersion equalizer 200 or 300 to coarsely compensate large chromatic dispersion. The coarsely dispersion-compensated signals 111 and 112 are fed into an adaptive butterfly FIR filter 113 with relatively small tap number to undergo chromatic dispersion compensation, polarization mode dispersion compensation and polarization demultiplex accurately. The adaptive butterfly FIR filter 113 is controlled by a relevant algorithm, and its tap coefficient is adjusted dynamically so that channel variation can be monitored and compensated in real-time. The output signals 114 and 115 of the adaptive butterfly FIR filter 113 are fed into frequency offset compensator 118 and 119 respectively, while a frequency offset monitor 116 estimates the difference between the carrier frequency of the received optical signal 101 and frequency of the local oscillation laser according to the signals 114 and 115, and inputs the difference into the frequency offset input ports of the frequency offset compensators 118 and 119 respectively. After the frequency offset compensation, the output signal 120 of the frequency offset compensator 118 and the output signal 121 of the frequency offset compensator 119 are phase-recovered via phase recovery units 122 and 123 respectively, resulting in phase-recovered signals 124 and 125. Finally, the signals 124 and 125 are decided and decoded by the deciding and decoding unit 126 and 127 respectively, resulting in data 128 and 129.
The chromatic dispersion equalizer 200 (or 300) in FIG. 1(a) is for the purpose of coarse compensation of dispersion, so that remaining dispersion comes within the compensation scope of the adaptive filter 113. The chromatic dispersion equalizer can process the signal in either time domain or frequency domain.
The structure of the time domain chromatic dispersion equalizer 200 is illustrated in FIG. 1(b). The input signals 109 and 110 of the time domain chromatic dispersion equalizer 200 are fed into filters 203 and 204 respectively, resulting in equalized signals 111 and 112. Since dispersion amount to be compensated is generally large, the filters 203 and 204 may have taps up to tens or even hundreds, and thus are called long FIR filters. A time domain equalizer coefficient storage unit 201 stores filter tap coefficient values 202 corresponding to groups of different dispersion compensation amount. The time domain equalizer coefficient storage unit 201 inputs the tap coefficient value 202 into the filters 203 and 204. Since the dispersion value of a fiber link will not change substantially for a long time, it is not necessary for the filters 203 and 204 to update the coefficient for a long time after reading it from the time domain equalizer coefficient storage unit 201.
The time domain chromatic dispersion equalizer 200 actually performs discrete convolution operation on a discrete time signal, and its algorithm is a relatively complex one. By using fast Fourier algorithm to convert a time domain signal into frequency domain signal for processing, it is possible to substantially reduce the operation amount. FIG. 1(c) illustrates the structure of a frequency domain chromatic dispersion equalizer 300 based on this idea. In the frequency domain chromatic dispersion equalizer 300, fast Fourier transformers 303 and 304 transform the input time domain digital signal sequences 109 and 110 into frequency domain respectively, resulting in frequency domain signals 305 and 306, which are input to frequency domain equalizers (e.g. frequency domain filters) 307 and 308 respectively. In the frequency domain equalizers 307 and 308, the frequency domain signals 305 and 306 are multiplied with an inverse function of the dispersion transfer function which uses the equalizer coefficient stored in the frequency domain equalizer coefficient storage unit 301, resulting in dispersion-compensated frequency domain signals 309 and 310 respectively. Finally, the signals 309 and 310 are converted back to time domain by fast inverse Fourier transformers 311 and 312, outputting dispersion-compensated time domain signals 111 and 112. The calculation of linear convolution by fast Fourier transform belongs to prior art.
In the course of implementing the present disclosure, the inventors found the performance of the conventional optical coherent receiver is not satisfactory.
The following reference documents of the present disclosure are incorporated herein by reference, as if they are described entirely in the present disclosure:
1. Chinese patent application No. 200810090227.4 entitled “frequency offset detection apparatus and method used in digital coherent optical receiver”.
2. Chinese patent application No. 200810108921.4 entitled “filter coefficient adjusting apparatus and method”.